Mechanical ventilation has long been used to support lung function in a patient, and entails the use of warmed and moistened fresh air, oxygen, or an oxygen-containing gas (such as an anesthetic gas) supplied to the patient under pressure for oxygenating the patient's lungs. One known method of mechanical ventilation is intratracheal ventilation (ITV). ITV involves the delivery of a warmed and well-humidified gas at or near the patient's carina (the fork of the trachea leading to the bronchial tubes). The gas is delivered through a ventilator tube positioned in the trachea. The ventilator tube can be a catheter, a tracheal tube, an endotracheal tube or the like, and the oxygen-containing gas can be supplied through it on either a continuous or periodic basis. In the former case, the ventilator tube can be supplied with a positive end expiratory pressure (PEEP) valve, whose periodic activation allows air from the lungs (enriched in carbon dioxide) to exit the patient's body.
While they do provide some degree of oxygenation when initially employed, the continued use of mechanical ventilators for ITV is subject to significant drawbacks. For example, mechanical ventilators have traditionally been required to deliver high gas pressures, in order to achieve adequate oxygenation. There are several reasons for this. When a mechanical ventilator is first installed, the interior of the ventilator tube usually contains just air, rather than an oxygen-enriched gas; when activated, this air is pushed into the lungs, while the oxygen-enriched gas remains outside the lungs. The alveoli in the patient's lungs expire carbon dioxide, and the expired carbon dioxide builds up in the lungs after several ventilation cycles, because the high pressure of the oxygen-enriched gas--and any straight air between the gas and the carbon dioxide in the lungs--prevents at least some of the carbon dioxide from escaping the lungs. The elevated level of carbon dioxide leads the health practitioner to supply even higher pressures of the oxygen-enriched gas to the ventilator tube. After a time as short as only a few hours, the patient's lungs may suffer damage from two sources, specifically, the elevated ventilator pressure, and the build-up of carbon dioxide. The result can be hypoxia, respiratory acidosis, hypercarbia, iatrogenic lung damage, pulmonary hypertension, overinflation, and/or pulmonary parenchymal injury. These problems can even be severe enough to result in the death of the patient. Further, while these problems are more likely to occur and to be more severe in patients with significantly impaired lung function (such as pediatric patients and patients who have undergone a partial lung removal), these problems can even occur in patients with healthy lungs.
Another known method of mechanical ventilation is intratracheal pulmonary ventilation (ITPV). ITPV similarly involves the delivery of an oxygen-containing gas at or near the patient's carina. The gas is continuously supplied either at a constant pressure, or by pressure pulses at a frequency of about 1 to 50 cycles per second. One drawback to this method is that carbon dioxide outflow is periodic or intermittent, controlled by relatively complex valve and timer mechanisms. As a result, the potential remains for an inadequate expiration of carbon dioxide and a resultant progressive build up of the carbon dioxide level in the patient. Additionally, the cyclical peak pressures typically employed in this method are high, often significantly higher than the pressures employed in ITV. Accordingly, all of the problems which may be encountered in the use of ITV may also be faced during the use of ITPV.
One solution to these and other problems has been the Kolobow reverse thrust catheter, such as disclosed in U.S. Pat. No. 5,186,167 (Feb. 16, 1993). The disclosure of that patent is expressly incorporated by reference herein. By way of summary, the Kolobow device has a catheter preferably contained in a tracheal or endotracheal tube. The catheter includes a plurality of ports through its distal end, the distal end of the catheter being positioned at or near the patient's carina. Air supplied through the catheter diffuses transversely through the ports, creating zones of sub-atmospheric pressure which facilitate removal of carbon dioxide-laden air from the patient's lungs. The particular embodiment shown in FIG. 3C in the patent includes a tubular portion 19 on the catheter tip 16 which defines an annular exit port 17, directing the flow of air and oxygen in a direction opposite the distal end 18 of the catheter. The patent notes at column 8, lines 4 through 14, that while it is preferred that the exit port is annular, it is not necessary to employ a port having the specific shape of an annulus; rather, the essential feature is to provide a means which directs the air and oxygen in a direction opposed to the distal end 18 of the catheter tip 16. Although not described in such terms in the patent, the low pressure zones are produced by the well-known venturi effect. For convenience, since the flow creating the venturi effect is directed opposite to the incoming flow of air and oxygen, devices of this type will be referred to herein as "reverse venturi devices."
The Kolobow or reverse venturi device functions quite well to move carbon dioxide-laden air out of the patient's lungs through the tracheal or endotracheal tube, thus improving oxygenation in comparison to that achieved with other ITV and ITPV devices. Moreover, this improved oxygenation is achieved at pressure levels significantly lower than the pressures employed in other ventilation devices, substantially reducing the risk of trauma to the patient from elevated ventilation pressures.
Unfortunately, the Kolobow or reverse venturi device (at least, insofar as actually constructed in practice) potentially presents a different risk to the patient. In the device, the catheter tip 16 causing the venturi effect is generally of solid material and is pressed over and attached to the distal end of a hollow and flexible catheter. As a practical matter, such attachment is problematic, due to the nature of the materials used and the small dimensions encountered. The result is that the catheter tip 16 may loosen and separate from the distal end of the catheter, and be deposited in one of the bronchi or lungs of the patient. As a result, the catheter tip 16 may damage the tissue of the lung and create complications such as fluid pockets, infections and patient discomfort. Furthermore, the separated catheter tip 16 may require surgical removal, and the complications attendant to lung surgery.